The invention relates to multi-carrier digital transmission systems and has particular relevance to discrete multi-tone or orthogonal frequency division multiplexed systems for use over digital subscriber lines or radio broadcast systems.
Digital subscriber line technologies, commonly termed xDSL enable high-speed digital data to be transmitted down an ordinary phone line. The modulation scheme standardised for asymmetric DSL (ADSL) and proposed for very high speed DSL (VDSL) is discrete multi-tone modulation DMT. In this scheme, several carriers are quadrature amplitude modulated (QAM) at the same time and added together. Modulation can be achieved by performing an inverse fast Fourier transform (IFFT), with fast Fourier transform (FFT) used for demodulation. The output from one IFFT calculation is termed a discrete multi-tone symbol and is sent over the channel after conversion to an analogue signal. A problem with normal telephone lines, which often comprise a simple twisted pair, is the frequency dependent attenuation and phase shift of a transmitted signal which result in the time dispersion or spread of the signal in time. This manifests itself as interference between adjacent symbols as one symbol is spread into a following symbol. The interference in one symbol is a combination of the interference due to a previously transmitted symbol, which is correctly termed the intersymbol interference ISI, and the interference due to the symbol itself, or the intercarrier interference. For the purposes of this document, no distinction will be made between the sources of the interference, and the term intersymbol interference or ISI will be used to designate the total interference experienced by a symbol. ISI can be viewed as a transient or decaying xe2x80x98tailxe2x80x99 generated at the discontinuity where consecutive symbols meet. Conventionally, the effects of ISI are mitigated by providing a guard interval in front of each symbol. The guard time typically contains a cyclic extension of the symbol. Specifically, a copy of the end of each symbol is added to the beginning of the symbol in the form of a cyclic prefix. The carriers are continuous from the beginning of the cyclic prefix to the end of the symbol. Thus any interference will be generated at the discontinuity between the start of the cyclic prefix and the end of the previous symbol. The lengths of cyclic prefixes vary according to the application, but typically consist of no more than 10% of the symbol. Longer guard intervals are unfavourable because they introduce a bandwidth penalty. If the dispersion on the channel is not too severe, the ISI transient generated at the boundary between symbols will terminate within the cyclic prefix, leaving the subsequent symbol intact. However, the impulse response of the channel, which includes the effects of filters in the transmitter and receiver, can be very long, and often exceed the guard interval. Residual intersymbol interference will then occur which can severely impair the quality of the received signals.
In the article xe2x80x9cResidual ISI Cancellation for OFDM with Applications to HDTV Broadcastingxe2x80x9d D. Kim and G. Stxc3xcber, IEEE Journal on Selected areas in Communications Vol. 16, No. 8, October 1998, a technique for cancellation of residual ISI is discussed. An algorithm is proposed for removing the interference generated between consecutive symbols transmitted on a channel. This includes the interference caused by the previous transmitted symbol, i.e. the inter-symbol interference (ISI), and the disturbance due to symbol itself, i.e. the inter-carrier interference (ICI). The determination of interference requires knowledge of the transmitted symbols. This is achieved by making decisions about the transmitted symbols utilising the received, decoded symbols that have been corrupted by the channel, with knowledge of the channel response. The estimated symbols are then converted back to the time domain using IFFT, and the ISI determined and removed using the algorithm. The residual symbol is then reconverted to the frequency domain using FFT and the decisions made. An iterative process then follows to remove the ICI. Since the decisions on the transmitted symbols may initially be erroneous, an iterative process is required to accurately determine the interference. This necessarily entails a very large number of calculations, so that the process as a whole demands very high processing power.
Time domain equalizers TEQ are also used in the art to mitigate the effects of ISI between symbols transmitted over a distorting channel. A time domain equalizer is constituted by a filter, generally a finite impulse response (FIR) filter and has the effect of shortening the impulse response of the channel. This can be achieved, for example, by cancelling the poles in the channel transfer function. By using a suitable algorithm, the channel impulse response can be made shorter than the cyclic prefix utilised. However, a drawback of TEQs is that both the noise and the signal are filtered. When a TEQ cancels the poles in the channel transfer function it will also attenuate some signal frequencies and amplify noise at other frequencies. The noise will leak into the side lobes of the fast Fourier transform in the receiver and degrade performance. Hence adapting the TEQ to minimise ISI will generally result in a sub-optimal signal to noise ratio.
There is thus a need for a system that reliably mitigates the effects of intersymbol interference while leaving signal information undisturbed but is simple enough to be implemented in a wide range of applications.
In a multi-carrier transmission system wherein digital symbols including a cyclic symbol prefix are transmitted over a transmission medium to a receiver, intersymbol interference is compensated for by generating an estimate of the ISI tail and subtracting this from the received signals. This is achieved by subtracting the symbol prefix from the beginning of each received symbol to obtain a portion of a transient intersymbol interference signal generated during transmission and using the transient portion with a filter function adapted to generating an estimate of the full transient signal. This transient signal is then subtracted from the received symbol to correct the same. The filter function may be preceded by a processing arrangement for generating the initial conditions of the filter function. The filter function is then used to generate the full transient signal when input with a predetermined value. The processing arrangement may be a second filter or be adapted to perform calculations.
The filter function may alternatively be an adaptive filter function that is configured prior to use with a training sequence. The adaptive filter function may also be adjusted using each received symbol to generate an error update signal.
The specific compensation for intersymbol interference in accordance with the invention means that the signal can be filtered separately to remove noise due to other sources in the normal way. This allows each process to be optimised without having a detrimental effect on the other. Moreover, the processing power required for this compensation is relatively small, since it requires the generation of a transient symbol using only the portion of the actual transient signal contained in the cyclic prefix and the cancellation of the interference by subtracting the generated signal from the beginning of the received symbol.